1. Field of Invention
This invention relates to tap changers used on transformers to select the output voltage from the transformer and, more particularly, relates to a combined solid-state and mechanically-switched transformer tap which provides fast, on-load switching and very fine voltage control.
2. Description of Related Art
Electric transformers utilize the principle of electromagnetic induction to step up or down a particular voltage level to a higher or lower voltage level. Electromagnetic induction induces a voltage on a conductor which is placed in a varying magnetic field. If the conductor is in the shape of a coil, the voltages induced on each turn of the coil are cumulative and therefore the voltage output is proportional to the strength of the magnetic field and the number of turns in the coil. Commercial transformers produce the varying magnetic field by applying an alternating current (AC) to an input or primary coil. By placing a magnetic core within both the primary or input coil and the output or secondary coil, the magnetic field is effectively coupled to the secondary or output coil and a voltage proportional to the number of turns in the secondary coil is produced. Since the amount of magnetic flux generated by the primary coil is proportional to the number of turns in that coil and the voltage produced by the output or secondary coil is proportional to the magnetic flux surrounding the secondary coil, the output voltage of the transformer is equal to the input voltage times the ratio of the number of turns in the input coil over the number of turns in the output coil provided losses are neglected. Thus by changing the ratio of input or primary turns to output or secondary turns, the ratio of input to output voltage can be changed thus controlling or regulating the output voltage of the transformer. Changing taps on a transformer regulating winding has long been used to control voltage magnitude, voltage phase angle, or both in electric power feeder circuits. Basically, the tap changer selects which turn in the secondary coil will be connected to the load circuit thereby changing the ratio of turns in the transformer and regulating the output voltage.
Changing taps can be accomplished with the transformer either on-load or off-load. Off-load changes are accomplished by using breakers to isolate the transformer and then manually switching the output connection from one tap to another. Such tap changes have been made under load with mechanically-switched devices or load tap changers (LTC). On-load tap changing requires that the regulating coil circuit be maintained while switching from one tap to the other. This is accomplished by a combination of selector switches and a diverter switch. One selector switch is positioned at the tap which is currently in service. The other selector switch is positioned and connected to the tap which will be put in service. The diverter switch has two main contacts each of which is connected to a selector switch. In addition, the diverter switch has two transition contacts, each of which is connected through a resistor or reactor to a selector switch. As the diverter switch is thrown from one main contact to the other, the transition contacts insert the resistors or reactors in the circuit until the switch makes contact with the second main contact. Thus, the regulating coil is maintained continuously in the circuit and circulating currents between the two taps are dissipated by the resistors or reactors. The time required for the complete tap change is on the order of 1 to 2 seconds. For damping electro-mechanical rotor oscillations in electric power grids and for compensating sudden load changes due to faults, high-speed tap-changing after one-cycle of power frequency is needed to stabilize power supply networks. In addition, the use of mechanical contacts in the diverter switch invariably leads to some arcing which in turn contaminates the insulating oil of the transformer.
Efforts to speed-up the operation of on-load tap changers and to eliminate arcing have included the use of solid-state switches such as thyristors to replace the diverter switch. U.S. Pat. No. 5,006,784 issued to Ernst Sonntagbauer on Apr. 9, 1991 describes such a system and is incorporated by reference herein. The use of solid-state switching can reduce the time required to accomplish a tap change to 120 to 200 microseconds. However, the system described in the Sonntagbauer patent still contains a number of significant drawbacks. Voltage or phase shifting regulation by tap changing is limited to changes between fixed tap positions which are determined during the design of the transformer. These tap positions generally provide for transformer output changes between 2% and 5% of the transformer nominal rating. Modulation between these steps is not possible. In addition, the thyristors used must be capable of withstanding full load current which increases initial cost and reduces the effective service life of the thyristors. Finally, on existing transformers, the mechanical diverter switch must be replaced with a thyristor network at considerable expense and labor.
One approach to overcoming these limitations is disclosed in "Flexible ac Transmission Systems (FACTS): Scoping Study" EPRI Report EL-6943 published September 1991, and subject in U.S. patent application Ser. No. 07/742,859, commonly assigned to the assignee of the present application and incorporated by reference herein. In that patent application, the use of a variable susceptance device in parallel with the series winding of a phase shifter is used to modulate the phase shifting capability of the unit between that provided by the discrete tap positions. In addition, the use of a thyristor valve connected to one of the unused taps on the regulating coil is placed in parallel with the LTC to effectively vary the tap position by discrete taps.
However, difficulties still remain with this scheme. The tapped voltage change following operation of tile thyristor-augmented reversing switch is equal to the total voltage rating of the regulating winding. The voltage rating of each thyristor switch is also equal to the total voltage rating of the regulating winding. The step change when switching can be too large to damp network oscillations. Also, a mechanical bypass switch is required to eliminate the nominal power losses which are roughly proportional to switch voltage rating. The tapped voltage change of the no-load thyristor tap-changer in parallel with LTC is still limited to a single step. While the size of the step can be an integer number of taps determined by mechanical selection, tile change in step size is too slow in speed to be useful for modulation. Also, because of the parallel connection of LTC and thyristor switch there is a possibility of circulating current when the thyristor switch is closed at a large change in tapped voltage from that established by the LTC. Such a circulating current would need to be limited by impedance or interrupted by a circuit breaker.